The adult pattern of axonal connections from the eye to the brain arises during development through the refinement of a roughly ordered set of connections. In the chick visual system, refinement normally results in the loss of the ipsilateral retinotectal connections. Inhibition of nitric oxide synthesis reduced the loss of these transient connections. Because nitric oxide is expressed by tectal cells with which retinal axons connect and because reduction of nitric oxide synthesis by tectal cells resulted in a change in the connections of retinal axons, nitric oxide probably serves as a messenger from tectal cells back to retinal axons during development.